An optical sensor system may comprise a Fabry-Perot structure. A Fabry-Perot structure comprises two reflective surfaces spaced apart a distance from each other.
The article entitled “Interferometric Fiber Optic Sensors” by Byeong Ha Lee et al. ISSN 1424-8220, describes a Fabry-Perot interferometer sensor. Interference occurs due to multiple superpositions of both reflected and transmitted beams at two parallel surfaces. A reflection or transmission spectrum of a Fabry-Perot interferometer can be described as a wavelength dependent intensity modulation of an input light spectrum. This wavelength dependent intensity modulation is mainly caused by an optical phase difference δFPI between two reflected or transmitted beams:
      δ    FPI    =                    2        ⁢        π            λ        ⁢    n    ⁢                  ⁢    2    ⁢                  ⁢    L  
wherein λ is the wavelength of incident light, n is the refractive index of cavity material or cavity mode, and L is a physical cavity length. In the thus modulated spectrum, maxima and minima occur at particular wavelengths when both beams are in phase and out-of-phase, respectively. A perturbation may cause a variation of an optical path length difference in the interferometer and thus a variation of the optical phase difference δFPI. For example, applying longitudinal strain to the Fabry-Perot interferometer sensor, changes the physical cavity length L or the refractive index n of the cavity material, or both. The variation of the optical phase difference δFPI causes a shift of the reflection and transmission spectrum of the Fabry-Perot interferometer sensor. By measuring this spectral shift, the longitudinal strain applied on the Fabry-Perot interferometer sensor can be quantitatively obtained.
International patent application published under number WO 2011/069013 A1 describes a spatial Fourier transform spectrometer that includes a Fabry-Perot interferometer having a gap that spatially varies in a direction orthogonal to the optical axis The Fabry-Perot interferometer creates an interference pattern from input light. A processor is configured to process an interference pattern image to determine information about the spectral content of the input light.
US patent application published under number US 2005/046862 A1 describes a fiber optic sensing device that uses a Fabry-Perot cavity to sense a physical parameter. The cavity modulates incident polychromatic light. The spectrum is analyzed in a signal processing unit which normalizes the spectrum and determines the phase of the modulated signal.
German patent application published under number DE 101 45 912 A1 describes a system in which a low coherence light of wide range is fed into a Fabry-Perot load cell having a measured clearance varied in response to physical quantities such as force and pressure or the like to modulate its wavelength.
International patent application published under number WO 2006/092052 A1 describes a Fabry-Perot optical sensor for sensing a parameter such as pressure or the like.